The compound 2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-b][1,5]benzodiazepine is named olanzapine according to the U.S.A.N. It is known as an anti-psychotic agent. Form I of 2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-b][1,5]benzodiazepine, having m.p. 195° C., is used in pharmaceutical formulations. A process to prepare olanzapine is disclosed in U.S. Pat No. 5,229,382, which is incorporated herein by reference. The last step of the reaction disclosed in the patent comprises mixing 4-amino-2-methyl-10H-thieno[2,3-b][1,5]benzodiazepine and 4-methylpiperazine and refluxing in a suitable organic solvent to yield the desired Form I. It has been found that olanzapine prepared according to the process of the '382 patent is contaminated with olanzapine Form II as an impurity. Accordingly there is a need for a process to prepare olanzapine Form I free of the Form II impurity.